1. Field of the Invention
This invention relates to an improvement of a prehardened steel used for manufacturing plastics molds.
2. State of the Art
To date general structural steels (for example, S55C) and medium or low carbon steels (typically, SCM445) have been used as the material for manufacturing plastics molds, particularly injection molds, to produce relatively large-sized moldings.
In a mold fabrication for which these materials are used, the circumstances are such that a mold on the way to fabrication must be repaired, so often, through build-up welding due to working errors or a design changes. For welding repair, a preheating (250.degree. to 350.degree. C.), and further a postheating, as occasion demands, will be necessary for prevention of weld crack.
However, the problem is that an exclusive heating furnace will be prepared preferably for ensuring a uniform heating and that the larger a mold is, the longer the time is required, and a welding work for the pieces of high temperature involves, as a matter of course, a lowered working efficiency. Weld cracks are quite unavoidable from carrying out welding on the insufficiently preheated molds, which are no more to attain the purpose, and there may be a case, still worse, where an excessive crack is caused thereon to necessitate refabrication consequently.
Besides, the steel for plastics molds must be ready for hardening, uniform in hardness at every sections, less in segregation and superior in both mirror finishing and crimping workability, and also satisfactory in machinability.